1. Field of the Invention
This present invention relates to an apparatus for and a method of generating electrical power in a downhole environment. It may be used in wireline applications, measurement-while-drilling (MWD) applications, and in a producing borehole.
2. Background of the Invention
In underground drilling applications, such as oil and gas exploration and development, a borehole is drilled through a formation deep in the earth. Such boreholes are drilled or formed by a drillbit connected to an end of a series of sections of drill pipe, so as to form an assembly commonly referred to as a “drillstring.” The drillstring extends from the Earth's surface to the bottom of the bore hole. As the drillbit rotates, it advances into the earth, thereby forming the borehole. In order to lubricate the drill bit and flush cuttings from its path as it advances, a high pressure fluid, referred to as “drilling mud,” is directed through an internal passage in the drillstring and out through the drill bit. The drilling mud then flows to the surface through an annular passage formed between the exterior of the drillstring and the surface of the bore.
The distal or bottom end of the drillstring, which includes the drillbit, is referred to as a bottomhole assembly (BHA). In addition to the drillbit, the BHA often includes specialized modules or tools within the drillstring that make up the electrical system for the drillstring. Such modules often include sensing modules, a control module and a pulser module. In many applications, the sensing modules provide the drillstring operator with information regarding the formation as it is being drilled through, using techniques commonly referred to as “measurement while drilling” (MWD) or “logging while drilling” (LWD). For example, resistivity sensors may be used to transmit and receive high frequency signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor.
In other applications, sensing modules are utilized to provide data concerning the direction of the drilling and can be used, for example, to control the direction of a steerable drillbit as it advances. Steering sensors may include a magnetometer to sense azimuth and an accelerometer to sense inclination. Signals from the sensor modules are typically received and processed in the control module of the downhole tool. The control module may incorporate specialized electronic components to digitize and store the sensor data. In addition, the control module may also direct the pulser modules to generate acoustic pulses within the flow of drilling fluid that contain information derived from the sensor signals. These pressure pulses are transmitted to the surface, where they are detected and decoded, thereby providing information to the drill operator. In view of the limited bandwidth of telemetry channels available in MWD environments, it is common practice to have a downhole processor that processes the measurements made by the sensors and also controls the direction of drilling.
It will be appreciated that the sensors and processors require a considerable amount of electrical power. In addition, power may also be required for drilling operations over and above the power of the rotating drillstring.
After the well has been drilled, additional measurements are made using sensors conveyed on a wireline or coiled tubing. These sensors are used for obtaining additional measurements of properties of the earth formation. Power requirements for wireline applications are usually met by transmitting power through the wireline. There are certain applications that will be discussed later that require high levels of power. It will be appreciated that when power is transmitted through a wireline that may be several kilometers in length, cable resistance can become an important limitation on the amount of power that can be transmitted downhole. For these high power requirements, it would be desirable to have an auxiliary power source downhole.
The control of oil and gas production wells constitutes an important and on-going concern of the petroleum industry. Production well control has become particularly important and more complex in view of the industry wide recognition that wells having multiple branches (i.e., multilateral wells) will be increasingly important and commonplace. Such multilateral wells include discrete production zones which produce fluid in either common or discrete production tubing. In either case, there is a need for controlling zone production, isolating specific zones and otherwise monitoring each zone in a particular well. As a result, the methods and apparatus for controlling wells are growing more complex and in particular, there is an ever increasing need for downhole control systems which include downhole computerized modules employing downhole computers (e.g., microprocessors) for commanding downhole tools such as packers, sliding sleeves and valves. An example of such a sophisticated downhole control system is disclosed in U.S. Pat. No. 5,732,776 to Tubel et al., which is assigned to the assignee hereof and incorporated herein by reference. Tubel discloses downhole sensors, downhole electromechanical devices and downhole computerized control electronics whereby the control electronics automatically control the electromechanical devices based on input from the downhole sensors. Thus, using the downhole sensors, the downhole computerized control system will monitor actual downhole parameters (such as pressure, temperature, flow, gas influx, etc.) and automatically execute control instructions when the monitored downhole parameters are outside a selected operating range (e.g., indicating an unsafe condition). The control devices and the processors also require a reliable source of power.
A variety of methods have been used for downhole generation of power. U.S. Pat. No. 5,839,508 to Tubel et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, teaches the use of an electrical generator that produces electricity from the flow of fluids in a production well. U.S. Pat. No. 6,554,074 to Longbottom teaches the use of electrical power generation using lift fluid in a producing well. U.S. Pat. No. 6,717,283 to Skinner et al. teaches a generator that derives its power from changes in annulus pressure in a producing borehole. U.S. Pat. No. 6,253,847 to Stephenson discloses electrolytic power generation wherein the casing is used as an electrode. U.S. Pat. No. 6,011,346 to Buchanan et al. and U.S. Pat. No. 6,768,214 to Schultz et al. disclose the use of piezoelectric generation of electricity that ultimately derives power from the motion of flowing fluids in a producing well. One drawback of the methods that rely on fluid flow for electric power generation is that they obviously cannot be used for wireline applications. In addition, the power outputs are limited and, being mechanical devices, the efficiency is generally low.
U.S. Pat. No. 5,248,896 to Forrest having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, teaches the use of an electrical generator that is coupled to a mud motor. Like the other methods discussed above, such devices are inapplicable to wireline applications. In addition, the power output is limited by the rate of mud flow, and the efficiency is generally low. Furthermore, since the generator is coupled to the mud motor, electrical power is generated at the cost of power available at the drillbit.
One of the problems encountered in downhole applications is high temperatures. The rate of increase in temperature per unit depth in the earth is called the geothermal gradient. The geothermal gradient varies from one location to another, but it averages 25 to 30° C./km. Thus, at a well depth of 6 km, the temperature could be close to 200° C. Electronic circuitry and processors are usually not capable of operating above 175° C. Accordingly, there is extensive prior art in cooling methods for downhole use. Included in the cooling methods is thermoelectric cooling.
In the most general sense, thermoelectricity can be defined as the conversion of temperature differences to electricity and vice-versa. Two traditional examples of thermoelectricity are the Peltier-Seebeck effect (thermocouples) and thermionic conversion (heating a material to release electrons). A third, non-traditional example of thermoelectricity is thermotunneling in which electrons can quantum-mechanically tunnel from one unheated material to another when the distance between the two materials is small enough. FIG. 2a is a circuit representation of a thermionic cooler. FIG. 2b is a schematic representation of a thermionic cooler. A voltage source 205 is connected to a collector 201 and an emitter 207 of electrons 203. Under certain conditions, a temperature difference results due to heat 221 being extracted from the collector and the emitter is cooled.
U.S. Pat. No. 4,375,157 to Boesen, includes traditional thermoelectric coolers that are powered from the surface. The thermoelectric coolers transfer heat from the electronics area within a Dewar flask to the well fluid by means of a vapor phase heat transfer pipe. U.S. Pat. No. 5,931,000 and U.S. Pat. No. 6,134,892 to Turner et al. discloses a system in which traditional thermoelectric cooling is used as part of a cascaded cooling system.
Thermoelectric power generation uses the same principles as thermoelectric cooling and is illustrated in FIGS. 2c and 2d. Shown is an emitter 257 that is heated so as to have a higher temperature than the collector 251. Electrons 253 move from the emitter to the collector, generating a current that flows through the load 255. Unlike thermoelectric cooling, we are not aware of any prior art using thermoelectric power generation for downhole applications. A large part of the problem lies in the difficulty of fabricating thermoelectric power generators, their low efficiencies and relatively low power output.
It would be desirable to have a method and apparatus for generating electrical power downhole that is flexible, has high efficiency and is capable of high power output. The present invention satisfies this need.